Using a set of numerical experiments from 39 CMIP5 climate models, we project the emergence time for 4°C global warming with respect to pre-industrial levels and associated climate changes under the RCP8.5 greenhouse gas concentration scenario. Results show that, according to the 39 models, the median year in which 4°C global warming will occur is 2084. Based on the median results of models that project a 4°C global warming by 2100, land areas will generally exhibit stronger warming than the oceans annually and seasonally, and the strongest enhancement occurs in the Arctic, with the exception of the summer season. Change signals for temperature go outside its natural internal variabilities globally, and the signal-to-noise ratio averages 9.6 for the annual mean and ranges from 6.3 to 7.2 for the seasonal mean over the globe, with the greatest values appearing at low latitudes because of low noise. Decreased precipitation generally occurs in the subtropics, whilst increased precipitation mainly appears at high latitudes. The precipitation changes in most of the high latitudes are greater than the background variability, and the global mean signal-to-noise ratio is 0.5 and ranges from 0.2 to 0.4 for the annual and seasonal means, respectively. Attention should be paid to limiting global warming to 1.5°C, in which case temperature and precipitation will experience a far more moderate change than the natural internal variability. Large inter-model disagreement appears at high latitudes for temperature changes and at mid and low latitudes for precipitation changes. Overall, the inter-model consistency is better for temperature than for precipitation.

Table 1. Information regarding the 39 CMIP5 models and their experiments as applied in this study. The superscripts "a" and "b" after model names stand for historical run periods of 1860-2005 and 1861-2005, respectively, whilst "c" indicates the RCP8.5 run for 2006-2099, rather than the historical run period of 1850-2005 and RCP8.5 run period of 2006-2100 for other models.

Fig. 1 Time series for global mean annual surface temperature changes by a nine-year moving mean method, for all 39 models under RCP8.5 above the reference pre-industrial period. The vertical black dashed lines show the first year and the median year to reach the 4°C global warming among all the models. In the right-hand legend, the listed years are the timing of 4°C warming for individual models, and "N/A" indicates that a 4°C global warming is not projected to occur in the 21st century.

Fig. 2 In the left-hand parts, dotted areas indicate the signal, contour lines the noise, and shading the signal-to-noise ratio (SNR), for the global annual mean (a) surface air temperature and (b) precipitation [units for signal: °C and %; noise: °C and mm d-1; SNR: °C °C-1 and mm d-1 (mm d-1)-1, for (a) and (b), respectively] associated with a 4°C global warming, calculated by the median of the 29 models under RCP8.5. In the right-hand parts, the vertical solid lines represent the globally averaged value for each variable; the other dashed lines show the zonal mean changes for the three variables over the globe (black), over the land (red) and over the ocean (blue); and the shading indicates one standard deviation of the projections from the 29 individual models.

Fig. 5 Inter-model disagreement for annual (a) temperature and (b) precipitation, as measured by the ratio of the standard deviation of signals of individual models to the absolute value of the median signal of all 29 models.

Fig. 6 As Fig. 2 but for a 1.5°C global warming.

Fig. 7 Difference between the changes associated with a 4°C and 1.5°C global warming in (a) temperature and (b) precipitation. Dotted areas indicate the signal and shading shows the SNR [units for signal: °C and %; SNR: °C °C-1 and mm d-1 (mm d-1)-1, for (a) and (b), respectively].

World Bank, 2012: Turn down the heat: Why a 4\(\circ\)C warmer world must be avoided. A report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics. World Bank, Washington, DC.

49

World Bank, 2013: Turn down the heat: Climate extremes, regional impacts, and the case for resilience. A report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics. World Bank, Washington, DC.

Using a set of numerical experiments from 39 CMIP5 climate models, we project the emergence time for 4°C global warming with respect to pre-industrial levels and associated climate changes under the RCP8.5 greenhouse gas concentration scenario. Results show that, according to the 39 models, the median year in which 4°C global warming will occur is 2084. Based on the median results of models that project a 4°C global warming by 2100, land areas will generally exhibit stronger warming than the oceans annually and seasonally, and the strongest enhancement occurs in the Arctic, with the exception of the summer season. Change signals for temperature go outside its natural internal variabilities globally, and the signal-to-noise ratio averages 9.6 for the annual mean and ranges from 6.3 to 7.2 for the seasonal mean over the globe, with the greatest values appearing at low latitudes because of low noise. Decreased precipitation generally occurs in the subtropics, whilst increased precipitation mainly appears at high latitudes. The precipitation changes in most of the high latitudes are greater than the background variability, and the global mean signal-to-noise ratio is 0.5 and ranges from 0.2 to 0.4 for the annual and seasonal means, respectively. Attention should be paid to limiting global warming to 1.5°C, in which case temperature and precipitation will experience a far more moderate change than the natural internal variability. Large inter-model disagreement appears at high latitudes for temperature changes and at mid and low latitudes for precipitation changes. Overall, the inter-model consistency is better for temperature than for precipitation.

World Bank, 2012: Turn down the heat: Why a 4\(\circ\)C warmer world must be avoided. A report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics. World Bank, Washington, DC.

49

World Bank, 2013: Turn down the heat: Climate extremes, regional impacts, and the case for resilience. A report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics. World Bank, Washington, DC.